The present invention relates to a spring unit used for a mattress or a box spring.
A conventional spring unit used for a mattress or a box spring is known to have the following structure. Each of a plurality of main springs is made of a single wire. Each main spring has a straight rod portion and a pair of spring portions such that upper ends of the spring portions are coupled to two ends of the straight rod portion, respectively. The plurality of main springs are disposed on a rectangular base plate such that the straight rod portions thereof are at right angles to each other, forming a matrix. Lower ends of the spring portions are thus disposed around a periphery of the base plate and are fixed to the base plate. The straight rod portions of the main springs that are disposed in a matrix form are reinforced by intermediate support springs. Each intermediate support spring has a straight rod portion and a pair of spring portions in the same manner as the main spring. Upper ends of the spring portions are coupled to the straight rod portions of corresponding main springs, respectively, and lower ends thereof are fixed to the base plate.
In the conventional spring unit of this type, the spring portions of both the main springs and the intermediate support springs comprise torsion bar springs, respectively. A torsion bar spring deforms only slightly under a compressive load. However, when a compressive load is repeatedly applied to the torsion bar springs, a permanent set tends to occur. As a result, the conventional spring unit tends to lose its elasticity at a relatively early stage of use.